Increasing interest in automotive safety, as well as improving U.S. Federal automotive safety regulations, make evident the need for indication of low hydraulic brake fluid. Fluid level sensors previously used have included a float-activated switch. Others have used various types of electronic sensors. The float-activated switch suffers from the limited gravitational actuation force. The gravity-actuated switch in the corrosive environment created by brake fluid, could become seized beyond the ability of a gravity system to actuate it. The result is a failure in the hazard mode wherein an actual hazard exists but the sensor fails to indicate the hazard. Among the various types of electronic sensors, one type uses a thermistor suspended at the sensing level in the reservoir connected to a small source of electrical power. When the fluid level is above the thermistor, the fluid carries off the small amount of heat dissipated in the thermistor. When the fluid level is below the thermistor, resistance heating in the body of the thermistor causes the thermistor body temperature to rise beyond the highest probable ambient temperature. Thermistors have a very strong negative temperature coefficient of resistivity. As the thermistor temperature increases, the thermistor electrical resistance decreases sharply. External electrical circuits detect the change in resistance and thereupon energize warning devices. Failure in the electronic sensor or its associated circuits can be either in the fail-hazard (actual hazard, safe indication) or in the fail-safe (actual safe, hazard indication) modes.
The prior art fails to disclose a brake fluid level warning indicator positively actuated by brake fluid pressure and which fails in the fail-safe mode.